Embodiments of the present invention generally relate to an apparatus and method for holding and conveying substrates in solar cell manufacturing processes.
Typically, a solar cell fabrication tool or apparatus requires high throughput to minimize the tool cost per watt. For example, an apparatus may run or process 1500 to 3000 wafers or substrates in one hour, with each substrate measuring approximately 156 mm on each side. To process such substrates, the apparatus must transfer the substrates from load-lock chambers, which are under vacuum conditions to a processing chamber. As used herein, the terms “load-lock” and “load-lock chamber” shall be used interchangeably and include a pressure-tight enclosure, in which the pressure in the enclosure can be reduced below atmospheric pressure.
Typical cluster tools have three chambers, which may be load-lock chambers, which are stacked on top of one another. Such tools only use one of these chamber to introduce substrates into a processing system. The remaining two chambers are used to bring substrates out of the system and to pre-heat substrates. With such tools, accessing any of the load-lock chambers releases the vacuum in all of the load-lock chambers and vacuum conditions must be resumed as soon as possible so processing can continue. After removal of the substrate, reduced pressure or vacuum conditions must be resumed and the entire chamber is pumped down. When substrates are rapidly removed, the chambers must be pumped down each time a substrate is removed. During pumping down, it is believed that throughput is limited because the pumping down process can take up to two minutes. This loss in time results in decreased efficiency and may result in up to 30 fewer substrates being processed per hour. To process as many substrates as possible, processing systems utilize rapid pumping to minimize the time lost during pumping down or forming a vacuum within the chamber.
Rapid pumping down or rapid pumping creates a number of issues. For example, thin and/or lightweight solar cells undergoing processing may shifting position in air currents or otherwise be damaged during processing. In addition, contamination in the load-lock chamber can re-deposit on the cell surface and/or water aerosols can form and deposit on substrates. To mitigate these issues, it has been found desirable to pump down at a slower rate of speed. Accordingly, there is a need for an apparatus and methods of using such apparatuses that can be used to pump down at a slower rate of speed, while maintaining a high level of efficiency to minimize tool cost per watt.